Power converters are used in a variety of portable electronic devices including laptop computers, cellular phones, personal digital assistants, video games, video cameras, etc. In addition, they are used in non-portable applications such as, for example, Light Emitting Diode (LED) driver converters. They may convert a dc signal at one voltage level to a dc signal at a different voltage level (this is a dc-dc converter), an Alternating Current (ac) signal to a dc signal (this is an ac-dc converter), a dc signal to an ac signal (this is a dc-ac converter), or an ac signal to an ac signal (this is an ac-ac converter). Typically, these types of converters include a diode bridge rectifier stage and a bulk storage capacitor which produces a dc voltage from an ac signal provided by an ac line. This dc voltage is further processed by a converter which generates an output signal that is applied across a load. In this configuration, the rectifying circuit only draws power from the ac line when the instantaneous ac voltage is greater than the voltage across the bulk storage capacitor, resulting in a non-sinusoidal current signal that has high harmonic frequencies. A drawback with this configuration is that the power factor or ratio of real power to apparent power is usually very low. Thus, the converter draws excess current but fails to use the excess current to perform or accomplish any circuit functions.
To address the power factor issue, integrated circuit manufacturers couple a Power Factor Correction (PFC) stage to the diode bridge rectifier, which improves the use of current drawn from the main ac line by shaping it to be more sinusoidal. Generally, power converters that include PFC stages are either two-stage power converters, i.e., a two-stage PFC architecture, or single stage power converters, i.e., a single stage PFC architecture. A converter having a two-stage PFC architecture allows for optimization of each individual power stage. However, this type of architecture uses a large number of components and processes the power twice. A converter having a single stage PFC architecture uses fewer components, processes the power a fewer number of times which can improve efficiency, and can be more reliable than a two-stage architecture. A drawback with the single stage architecture is that it has a large output ripple which is at twice the ac line frequency. The magnitude of this ripple can overdrive conventional feedback networks forcing them outside of the linear response region or degrade their ability to maintain a high power factor. A technique for smoothing out or decreasing the ripple is to couple a filtering capacitor having a large capacitance value to the output filter network. Although the large capacitance smoothes out the ripple in the current delivered to the load without interfering with the control loop, it uses electrolytic capacitors which are large, expensive, and degrade circuit reliability. In addition, the large capacitance slows the response time of the control loop resulting in excessive current which can overdrive and potentially damage an LED load. The excessive currents typically occur when the converter is first energized or if the input voltage changes rapidly.
Another approach to mitigate high output ripple involves slowing the response time of the LED current feedback signal. The slower response introduces a delay in the feedback signal which is no longer representative of the actual current at a given moment in time. A slow control loop is used to minimize the effect of phase delay in the LED current feedback signal and maintains stable operating conditions. This slow response limits the circuit in responding to changing power line conditions potentially creating an excessive LED current. Initial power up also creates excessive current due to overshoot which can damage the LEDS. Systems with a slow feedback response are also prone to flicker which is undesirable in light sources.
Accordingly, it would be advantageous to have a method and a circuit that provides a feedback signal to a switching power controller that represents the average load current without ripple or time delays thereby allowing a rapid response to changing operating conditions. It would be of further advantage for the power converter and method to be cost efficient to implement.